Systems incorporating beam placement sensors to align laser beams are well known in the art. Most are usable only with continuous wave lasers although some are usable with pulsed lasers. Some are usable only once and are destroyed in use whereas others are reusable a number of times.
One particular system of interest is that disclosed in U.S. Pat. No. 4,035,654 to Elmer which shows a silicon disc device for use solely with continuous wave lasers, not being usable with a pulsed laser. Disposed on one side of the disc are a plurality of thermocouples which respond to variations in temperature on the disc when a beam is passed therethrough. The thermocouples connect to circuitry which provides an indication of beam misalignment, indicating a null when the beam is centered. The Elmer system has several intrinsic limitations as compared to the instant invention.
There is a need for a good beam alignment detector for high power pulsed lasers such as CO.sub.2 lasers. One element that appears to be very desirable for such use and which is used in the preferred embodiment of the invention is silicon because it has very high damage threshold for wavelengths generated by CO.sub.2 lasers. Too, very importantly, the thermoelectric effect in silicon is higher than for most other materials. Silicon is also very inexpensive and the art of manufacturing devices from the material is highly developed. The absorption of beam energy by the detector can be controlled by introducing selected amounts of impurities into the silicon which, because it has been utilized in electronic devices for a long period of time, has well known properties. Therefore, for many reasons, silicon appears to be an excellent material for use in practicing the invention. Another desirable property of silicon is that it can be manufactured to be transparent; thus it can be located in the beam path during actual laser operation with inconsequential effects on the power of the beam at a target. Alternatively, a silicon disc can be painted to be reflective, utilizing for example 10% of the beam energy for beam alignment and reflecting 90%. It can also be coated with an opaque coating and removably positioned in the beam path. One particularly good material for coating the silicon is lithium fluoride which is 100% absorbing and has an extremely high damage threshold for CO.sub.2 laser radiation wavelengths.
It is important to note that the invention makes use of the pulsed heat diffusion properties of whatever material is selected for use as a detector. The material utilized in practicing the invention needs thermoelectric properties capable of generating a voltage from heat differences in the body of the device. The Elmer detector, although constructed of silicon, is utilized solely as a heat absorber, requiring thermocouples attached thereto. Electrical signals from Elmer's thermocouples, not electrical signals from the silicon itself, indicate temperature, the Elmer detector not utilizing the thermoelectric properties of silicon. This limits the time response of the Elmer detector and prevents its use in pulsed beam positioning. In Elmer's device, local thermal equilibrium must be established between the thermocouple heat detectors and the silicon disc. This could take as long as several seconds. In the instant invention the electrical signal follows the local silicon temperature in times of about 10.sup.-12 seconds making it ideal for pulsed beam positioning.